Milankovitch Cycles

General Cycles

Denton & Broecker (1990) - glacial/interglacial cycles exist on a regular cycles

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Suggests that there is a predictable force driving long term climate changes

Impact of Mountain Building

Stuff like Indian/Asian collision leading to Himalayan Plateau

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Leads to cooling but this is not on a regular cyclical pattern

Impact of Atmospheric Changes

E.g. Volcanic Eruptions

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Can alter the climate over relatively short timescales

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Also not regular cycles

Earth Orbit characteristics

Always rotating and tilted on axis

This changes the level of insolation with seasons etc.

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Axis always points in the same direction during an annual cycle (towards sun in NH summer and away in NH winter)

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Orbit is slightly elliptical due to gravitational pull of the gas giants pull it out of a perfect circle

Earth Orbit stages

Perihelion - closest to the sun

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Aphelion - further away from the sun

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Summer Solstice - hemisphere tilted towards sun → longest day of the year

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Equinox - direction of axis side on to sun → equal length of night and day

Eccentricity of Orbit

Change in the shape of orbit from almost circular to elliptical

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Causes slight change in total radiation received and summer/winter contrast

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100,000 year cycle

Obliquity

Variation in tilt of earth’s axis between 21.5° and 24.5°

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Changes the relative contrast between the seasons

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41,000 year cycle

Precession of the Equinoxes

Caused by a wobble in the earth’s axis

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Results in progressive change in direction in which the earth’s axis points

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Position of equinoes and solstices on the elliptical orbit gradually change over time

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Pole Star changes with precession.

4,000 years ago when Pyramids were built the star Thuban was the pole star. Now is Polaris

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21,000 year cycle

Impact of Milankovitch Cycles

Long term change in earth’s insolation

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Linked to the climate changes

Oxygen Isotope Stage 11 Problem

450,000 years ago

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One of the longest and warmest interglacial

Really warm sea surface temps and high sea levels

Warmest interglacial period in the last 500,000 years

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Doesn’t fit the model

100ka problme

Pacing of the ice age has shifted from a 41ka cycle to 100ka cycle

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Shift occurred around 800,000 years ago

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So must be other factors not just orbital impacts

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Obliquity was dominating now its eccentricity

Weird bc eccentricity changes solar radiation the least

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This could be bc ice age feedbacks etc.